Viral and extrachromosomal genomes are found in all types of cells, ranging from bacterial to human. The object of this research is the elucidation of the duplex and tertiary structures of the closed circular DNAs, which constitute an important subclass of these genetic elements. We further wish to inquire whether or not intercalative mutagenic agents react at different rates with closed and open DNAs, work which will complement the already known equilibrium preference for native closed DNA. The closed DNAs differ from all others in the possession of a tertiary structure, and we have designed experiments to measure the thermodynamic properties of the superhelix. We will determine the entropy and enthalpy of superhelix formation by measuring the binding of intercalative drugs at different temperatures. The results will be used to test the validity of various models of superhelical structure. Experiments are further proceeding to determine the in vivo effects of the mutagen ethidium bromide upon the extent of supercoiling and the generation of complex DNA in a model bacterial system, the plasmid DNA in E. coli 15. Finally, we will take advantage of sedimentation properties of closed DNA to detect and measure the extent of variations with solution environment of the duplex structure of both closed and open DNAs. Our previous work has shown that the PM-2 viral duplex is extensively unwound in aqueous chaotropic solvents, and further work in this area is expected to shed more light upon the factors which determine the fine structure of native DNA.